Description: The backbone of this program project is the Clinical Site Core which unites the overall aims of the proposal: 1) to isolate, characterize, and synthesize the bioactive substances in human milk (including glycoconjugates, antibodies, and anti-inflammatory factors) that protect the infant from enteropathogens; 2) to evaluate, through molecular-epidemiologic research, the protection of breast-fed infants from enteropathogens; 3) to determine, through randomized trials, the efficacy of specific bioactive substances identified in human milk to protect the infant from enteric infection and disease; and 4) to determine the effect of human milk on modulation of the immune response to enteric pathogens and vaccines. In the previous funding period of the previous application, the Clinical Site Core supported all projects. This included the sharing of demographic data and specimens, conducting field studies in Mexico (San Pedro Martir, Tlalpan) and Norfolk, specimen collection from Mexican and U.S. mothers, as well as biochemical, and biostatistical support. Field studies originating in 1987 in Mexico have enrolled in excess of 600 infants and 400 mothers. Additional studies (funded outside this project) enrolled over 600 infants in Norfolk. The Core maintained a database of all biological samples which was shared by all investigators. There have been over 2,800 milk, 1,600 blood, and 32,000 fecal samples collected, much of which has been depleted for analyses in the specific projects during 1993-8. Some of the remaining fecal samples can be used for the proposed projects, but there remain too few blood and milk samples and/or additional samples are needed to address newer aims in the current proposal.